As microwave plasma processing apparatuses generate plasma of a process gas using a microwave introduced into a processing container, there has been known a low pressure plasma type which generates plasma under a reduced internal pressure of a processing container and an atmospheric pressure plasma type which generates plasma under an atmospheric pressure.
As one example of the low pressure plasma type conventional technique, there has been proposed a plasma processing apparatus in which the arrangement and number of slots formed in the lengthwise direction of a wave guide are defined based on the relationship between a free space wavelength λ and a wave guide wavelength λg, and in which an impedance within the wave guide when viewed from a microwave power source is approximately equal to an impedance within the wave guide when viewed from its reverse direction. The proposed apparatus is advantageous in that a uniform plasma process can be performed for an object with a large area to be processed, such as a plastic film or the like. However, in the apparatus employing the low pressure plasma type, a dielectric plate is interposed between the wave guide and a processing container in order to keep the processing container at a low pressure. This dielectric plate may be effective in the respect of enhancing uniformity of plasma; however, it deteriorates the efficiency of energy use by absorbing some of the microwave. Accordingly, this leaves room for improvement for the purpose of processing the object with high-density plasma while minimizing energy loss.
In addition, it is effective to feed process gas without partiality for the microwave introduced into the processing container. However, in general, in a structure where the dielectric plate is interposed between the wave guide and the processing container, the process gas has to be directly introduced into the processing container. Moreover, a shower head cannot be formed on a ceiling of the processing container due to the existence of the dielectric plate so that a gas introduction part is limitedly disposed in a position separated from the wave guide (for example, a side wall of the processing container). Since the gas introduction part is restricted as such, it may be difficult to realize plasma uniformity in the processing container and process uniformity in the surface of the object.
As another example of the low pressure plasma type conventional technique, there has been proposed a plasma processing apparatus in which a wave guide propagating a microwave is inserted in a vacuum container. According to the apparatus proposed above, the wave guide is disposed within the vacuum container so that a dielectric member for maintaining a vacuum can be made small and thin, and an object with a large area to be processed can be uniformly processed. However, the apparatus has a double structure where the wave guide is disposed within the vacuum container which requires air-tightness, which may result in a complicated configuration of the apparatus which may make it difficult to realize. In addition, although the apparatus has no dielectric plate, the apparatus has a problem in that it is difficult to achieve plasma uniformity in the processing container and process uniformity on a surface of the object since a gas introduction part is formed on a side wall of the processing container which is separated from the wave guide.
As one example of the atmospheric pressure plasma type conventional technique, there has been proposed a plasma processing apparatus having a plasma generator including a slot antenna, an uniformity line which is connected at a right angle to a slot-formed surface of the slot antenna and brings a microwave into uniformity, and slits which are provided at a leading end of the uniformity line and emits the microwave. The plasma processing apparatus is configured to process an object to be processed, which is placed on the outside of the slits, with plasma under an atmospheric pressure by successively feeding process gas within a gap between the slits and the object to generate the plasma. Although this atmospheric pressure type plasma processing apparatus is advantageous in that it does not require a dielectric plate, it requires the slots for a wave guide and the slits for the uniformity line, and in other words, has a structure where two wave guides and two slots are arranged. Accordingly, this apparatus has a complicated structure and difficulty in controlling transmission of the microwave, and has a possibility of intermediate attenuation of the microwave due to occurrence of reflective wave. Therefore, the apparatus is not satisfactory in the respect of generating plasma with high efficiency.